engine : Rebalance status to show Started and Stopped At fields

Stopped At field is dispalyed in the rebalance status dialog once rebalance is stopped.

Change-Id: I2957ba2c4bd1f6deb1ef444e28e8afdd8896f0b4
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1022937
Signed-off-by: Anmol Babu <anbabu@redhat.com>